Zdoom, and thereby GZdoom, now support midi playback using fluidsynth. If you want to try this feature, you will need to get a copy of fluidsynth.dll compiled by Randy Heit.

To use the feature, unpack the above linked file into the same directory as you put your Zdoom and/or GZdoom SVN binaries. Then, start the game and go to the sound options menu and change the midi device option. There should be a listing for fluidsysnth. Select it and restart the sound.

With some sound cards (eg Creative cards) this should work "out of the box" with the cards' built-in sound font. However, other users may have to download and/or initialise a soundfont before using fluidsynth. The fluidsynth website has links to a few soundfonts which may be useful.

If you experience difficulties/need more help or wish to comment on fluidsynth support in some other way, the Zdoom forum would be the appropriate place.